Abstract: Procedure for the preparation of dioxopenicillanic acid derivatives and its salts pharmaceutically acceptable with general formula I, where, R is hydrogen, alkyl group containing 1 to 5 Carbon atoms or a residue of type --CH2R', where R' is hydrogen, halogen or a p-toluensulfonyl group. These are prepared by treatment of the compounds of general formula II, where R is as previously defined and X may be hydrogen or bromine, with a metallic reagent constituted by a mixture or alloy of copper and/or cobalt and/or manganese with iron and/or nickel in an aqueous/organic medium.These compounds are useful as inhibitors of beta-lactamase.